How is the Rf value of a substance calculated?

The Rf value, or retention factor, of a substance in chromatography is calculated as the ratio of the distance traveled by the substance to the distance traveled by the solvent front. This measurement is essential because it provides a way to identify and compare substances based on how far they move through the stationary phase relative to the solvent during the chromatographic process.

Specifically, the Rf value indicates how well a substance interacts with the stationary phase versus the mobile phase (the solvent). A higher Rf value generally indicates a greater affinity for the mobile phase, while a lower value suggests a stronger interaction with the stationary phase.

This method of calculation allows for consistency and comparability in analytical chemistry, as different substances will have distinct Rf values under the same experimental conditions, making it possible to identify and differentiate them in various samples.

The other choices do not accurately represent the relationship needed to compute the Rf value and would lead to incorrect interpretations of chromatographic results. For example, calculating Rf by swapping the distances or considering the total distances of phases does not reflect the fundamental principle behind how chromatography separates different substances. Therefore, the equation that divides the distance moved by the substance by the distance moved by the solvent is the correct one.